C++: dynamic_cast causes a SEGFAULT even when the object that is casted is not NULL. How can that happen?

6
votes

Suppose I have a class A and a class B that is derived from A. Now, I want to cast a const A* (called "a") to a B* using dynamic_cast (see below). If "a" really was a B*, then my resulting object pointer should be fine. If "a" was not a B*, then I will get NULL. 

const A* a = new B();
const B* b = dynamic_cast<const B*>(a);

For some reason, the dynamic_cast operation causes a SEGFAULT. How can that happen if "a" is NOT NULL? I guess that dynamic_cast will give me a NULL pointer if there were any conversion problems, instead of a SEGFAULT. I should only get a SEGFAULT if I am trying to access "b" and the dynamic cast was unsuccessful, right? I have not even tried to access "b" yet.

So, how can this happen? Is there anything that can cause dynamic_cast to SEGFAULT in the above code, that I am not aware of?

Thanks in advance :-)

EDIT: Running my actual program through GDB gives this output:

Program received signal SIGSEGV, Segmentation fault.
0x0000000000000000 in ?? ()
(gdb) where
#0  0x0000000000000000 in ?? ()
#1  0x00007ffff6c0e612 in __cxxabiv1::__dynamic_cast (src_ptr=<optimized out>, 
src_type=0x4fa6b0, dst_type=0x516bb0, src2dst=0)
at /var/tmp/portage/sys-devel/gcc-4.6.3/work/gcc-4.6.3/libstdc++-v3/libsupc++/dyncast.cc:61

The next line in the output just points to the line in my code where I do the dynamic casting.

3
c++segmentation-faultdynamic-cast
The problem is in the definition of A and B – with appropriate definitions your code will work. Therefore, post a minimal, complete code! – Konrad Rudolph
Since your example code is obviously not your real code, are you using dynamic_cast to get a reference type instead of a pointer type? In that case, an exception will be thrown. See here: ideone.com/uugF37 – Chad
Don't "suppose" code to us, write the minimal, compilable test code and show us. Otherwise you're asking us to use psychic powers to fix your code. – GManNickG

3 Answers

17
votes

Reasons which can cause a crash when using dynamic_cast

  • pointer points to a free memory block.
  • pointer points to a non-polymorphic type.
  • pointer points to an object with a polymorphic type but present in an external library compiled with RTTI disabled.
  • pointer points to a memory accessing which can cause protection exception (such as a guard page or inaccessible page).

Verify if one of these cases is applicable to you.

2
votes

For anyone else like me, you may have accidentally given the cast object the same variable name as the object you were casting it from!

A *name = new B();
B *name = dynamic_cast<B*>(name);

This is clearly wrong with such obvious code however out in the wild mistakes like this can be much harder to spot as variables are dispersed and casts are obfuscated!

0
votes

I got this one once... it was because I'd failed to initialise the pointer, and it was pointing at random garbage.